Concrete and other hydraulic mixtures used for construction rely primarily on the manufacture of Portland cement clinker as the main binder controlling the rate of development of mechanical properties. The manufacture of Portland cement clinker is energy intensive and releases large amounts of carbon dioxide into the atmosphere. To reduce the environmental impact of cement and concrete manufacture, supplementary materials with lower carbon dioxide footprint may be used to partially replace Portland cement clinker as the binder in hydraulic mixtures.
Large amounts of coal ash and other coal combustion products are generated worldwide from the burning of coal as fuel for electricity generation and other energy intensive applications. Unfortunately, the use of coal ash and other coal combustion products in concrete has many drawbacks. For example, addition of fly ash to concrete results in a product with low air entrainment and low early strength development. A critical drawback of the use of fly ash in concrete is that initially the fly ash significantly retards the development of mechanical properties of the concrete. Tests conducted by Ravindrarajah and Tam (1989, Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, SP-114, American Concrete Institute, Detroit, pp. 139-155) showed that the compressive strengths of fly ash concrete at early ages are lower than those for the control concrete, which is a general property of concrete or mortar when coal ash is added. Most of the reported studies tend to show a lower concrete strength due to the presence of coal ash.
Yet another critical drawback is that fly ashes can contain high levels of free carbon, measured as loss on ignition (LOI) or by other analytical methods, such as thermogravimetric analysis. The presence of excessive carbon levels inhibits the action of chemical additives used as air entrainment agents in concrete. Air entrainment in concrete is essential to ensure the durability of concrete under repeated freeze-thaw cycles. Hence, the use of combustion products in concrete is limited by the presence of free carbon.
In practice, these drawbacks prevent the economical use of coal ash at higher substitution levels than 25 to 30 percent in most construction concrete. As a result, a large amount of coal combustion byproducts are disposed of in landfills, at a high economical and environmental cost. Existing methods to beneficiate coal ash so as to make it suitable for other uses, such as in construction, generally do not enable 100 percent usage of coal ashes in beneficial applications. Furthermore, existing treatment methods commonly either use cost ineffective application of chemicals, or require treatment at a separate facility from where the coal combustion takes place, therefore incurring additional transportation costs and capital investments. Currently, most changes made to beneficiate coal combustion products are strictly related to the cleaning or sequestration of harmful chemicals within the coal combustion product. Some methods involve the use of external grinding facilities to reduce the particle size of combustion product particles. Other existing methods include carbon burn-out methods, utilizing the application of heat to the combustion product to reduce the levels of free carbon. Yet other methods utilize electrolytic methods to sequester carbon atoms. All of the aforementioned methods require high capital and ongoing costs in building and operating separate facilities.
The present invention has been developed in view of the foregoing and to remedy other deficiencies of the prior art.